In the past, oil-water separators have usually incorporated replaceable coalescer cartridges. While the coalescer cartridges are effective in removing the oil from the water, they have a relatively short service life particularly when dealing with large volumes of liquids.
Sand bed filters have been used in the past in municipal and industrial applications, primarily for the removal of suspended solids from water or other liquids. More recently, sand bed filters have been used in large industrial applications for the separation of oil from water. While sand bed filters are not as effective in removing the oil from water as a coalescer, they do have the advantage that they can be regenerated by backwashing.
The typical sand bed filter, as used in the past, is composed of a number of different layers or strata of particulate material. Materials such as anthracite, silica sand, and garnet sand, have been used in the past as the filtering media. The use of multimedia filters creates a need for extreme care in media selection and backwash design to prevent loss of media and intermixing of the layers during backwashing. This is particularly true when the filter is designed to separate oil from water, because the oil adsorbed on the particles adds buoyancy to the bed and the increased buoyancy during backwashing can cause loss of media and intermixing of the strata.